(a) Technical Field
The present invention relates to a shift control method for an automated manual transmission (AMT) in a hybrid vehicle, and more particularly, to a technology that improves gear engagement at the time of shifting a hybrid vehicle mounted with an AMT.
(b) Background Art
More recently, AMTs have been increasingly used to increase fuel efficiency, and improve power performance, and driver convenience in both hybrid and none hybrid vehicles. An AMT does not change gears automatically, but rather facilitates manual gear changes without the use of a clutch pedal. An AMT uses electronic sensors, pneumatics, processors and actuators to execute gear shifts on the command of the driver or by a computer. This removes the need for a clutch pedal which the driver otherwise needs to depress before making a gear change in a conventional manual transmission. The clutch itself on the other hand is actuated by electronic equipment which can synchronize the timing and torque required to make gear shifts quick and smooth.
Since a conventional automatic transmission, is equipped with a torque converter, the “drivability” of the vehicle is relatively high, but power transmission efficiency is quite low, and as a result, the fuel efficiency is low as well. On the other hand, an AMT has relatively small power loss, but the drivability is slightly lower than the conventional automatic transmission since the power transmission may be interrupted during gear changes.
Therefore, reducing a period of time that power is interrupted by shortening the shift time is of significant importance in designing an AMT. Further, since gears are disengaged and engaged during gear transgression, it is important to facilitate the disengagement and engagement of the gears accordingly for if the gear engagement fails, shifting and driving the vehicle becomes impossible
In particular, in the hybrid AMT vehicle, since a motor is connected together with a transmission input shaft, synchronization is difficult via only a synchronizer without the motor. Therefore, the rotational speed of the input shaft is increased close to the synchronization rotational speed by the motor and thereafter, synchronization is achieved by the synchronizer within a small speed difference range.
FIG. 1 shows a situation in which a hybrid AMT vehicle fails to engage in the proper gear during shifting on an uphill road and shows a graph corresponding to the engine speed and the transmission input shaft speed in addition to a graph of the motor torque according to the temporal change. Referring to the graphs of FIG. 1, for synchronization after disengaging a clutch, control following a motor speed is performed with a synchronization revolutions per minute (RPM) of a next target gear stage and thereafter, the motor torque is removed and the gear engagement is attempted.
However, in this case, as a vehicle speed is reduced on the uphill road, the speed of an output shaft is sharply reduced as well, and as a result, a control target rotational speed of an input shaft for synchronization with the speed of the output shaft is also sharply reduced. Since the inertia of the motor is still relatively larger than a typical disk clutch in the conventional art, the frictional force of the synchronizer can not bear the inertia in spite of a small speed difference of approximately 50 RPMs. Thus, synchronization is almost achieved during initial synchronization, but as time elapses, the gear engagement finally fails due to a synchronization collapse phenomenon in which the difference between the control target rotational speed of the input shaft and the input shaft speed is widened.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.